1. Field of the Invention
The present invention relates to a valve drive apparatus of an internal-combustion engine for realizing internal EGR.
2. Description of the Related Art
A reciprocating engine (internal-combustion engine) mounted on an automobile performs EGR (exhaust gas recirculation), that is, to recirculate part of engine exhaust gas to the intake side, as required, in order to improve the composition of exhaust gas and activate the catalyst earlier. Recently, as the EGR, aside from an external EGR which requires relay pipes around the engine, an internal EGR not requiring such pipes comes to be used. In the internal EGR, out of the engine combustion cycles (intake, compression, expansion and exhaust cycles), an intake valve is opened while an exhaust valve is opened in an exhaust stroke. By making use of the overlap period of exhaust and intake occurring at this time, part of exhaust gas is mixed into the air sucked in from the intake valve.
While the engine is running, at the time of idling or other low load time, the temperature of exhaust gas is low, and the catalyst does not function sufficiently. It is hence required to heat the catalyst to the active temperature in a short time. In such a case, by encouraging the EGR, it is known that the cylinder internal temperature is raised to elevate the exhaust gas temperature earlier. To enable such sufficient EGR, hitherto, the engine has been provided with a valve timing control device for shifting the cam phase to the advancing direction or retarding direction. In such a valve timing control device, by adjusting a phase difference between an open/close timing of the exhaust valve in the exhaust stroke and an open/close timing of the intake valve in the intake stroke, the overlap amount is increased, and the recirculation flow of exhaust gas is increased.
Besides, for example, in Jpn. Pat. Appln. KOKAI Publication No. 2000-345872, a valve timing control device designed for changing over the cams is disclosed. In this device, by changing over the cams for driving the exhaust valve or intake valve, the mode is changed to one in which an overlap amount is increased.
To increase the overlap amount, it is required to start releasing of the intake valve before the end point of the exhaust stroke. However, since the intake valve is released in the vicinity of the top dead center of a piston, when the overlap amount is increased, the intake valve interferes with the piston. Therefore, by the increase of the overlap amount alone, the EGR is limited, so that it is hard to enhance the effect of the internal EGR.